Improving the efficiency of thin film electroluminescent displays

Alternating current thin film electroluminescent (ACTFEL) devices have been used in applications requiring small area emissive displays with both high resolution (300 pixels per inch), and high intensity (>3000 fL). These requirements have been met by increasing the luminescent efficiency of ACTFEL devices via two independent techniques: (1) Alternative device geometry for enhanced out coupling of light. (2) Materials engineering for enhanced generation of light. ACTFEL devices are essentially thin film capacitors across which an alternating field is applied. In the `on state', the capacitance of the phosphor layer collapses due to the internal transfer of charge from one interface to the other. Power dissipation in the `on state' is therefore dictated by the drive voltage, drive frequency and the `on state' capacitance. For all of the L-V measurements presented, the device area and drive frequency were held constant, so that at a particular voltage, the relative luminance values are equivalent to relative luminous efficiency values. Hence the efficiency of an ACTFEL device has been demonstrated to be improved by both the utilisation of reflected lateral emission, and by the inclusion of a barrier layer in tandem with high temperature annealing.